Theoretical Investigation of Surface Reactions of Lactic Acid on MgO

Jan 15, 2013 - Lila B. Pandey and Christine M. Aikens* ... various sizes of magnesium oxide clusters (MgO)x (x = 2, 4, 6, 8, 9, 12, 15, 16, 18) is inv...
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Theoretical Investigation of Surface Reactions of Lactic Acid on MgO Clusters Lila B. Pandey and Christine M. Aikens* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States S Supporting Information *

ABSTRACT: Interaction of lactic acid with various sizes of magnesium oxide clusters (MgO)x (x = 2, 4, 6, 8, 9, 12, 15, 16, 18) is investigated. Density functional theory with the PBE functional and a polarized double-ζ basis set is employed to optimize the structures. For MgO dimer, optimizations are also performed at the PBE/DZP, PBE/TZP, and MP2/TZV(d,p) levels of theory, and single-point CCSD(T)/TZV(d,p) calculations are computed at the PBE/TZP optimized geometries. CCSD(T)/TZV(d,p) calculations show that the PBE/DZP results are off by no more than 5 kcal/mol. Dissociative adsorption of a proton bound to oxygen is possible for the carboxylic acid group, the hydroxyl group, and for a simultaneous interaction of the carboxylic acid and hydroxyl groups. Associative adsorption of various functional groups is also possible, although these represent higher energy structures. All of the adsorptions are calculated to be exothermic. Dissociative adsorption of the carboxylic acid group of lactic acid at the lowest coordinated magnesium sites is determined to be the lowest energy structure. Adsorption energies are found to decrease in magnitude as the size of MgO increases. The geometry of the magnesium oxide cluster changes to a nanorod-like structure when lactic acid interacts with higher coordinated magnesium atoms in double layer systems, but remains simple cubic when a third layer is added. The coordination environment rather than the size of the MgO cluster appears to dominate the adsorption energy when the size becomes larger than (MgO)12.

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occurs in a dissociative manner on MgO and ZnO surfaces.9,10 The monodentate, bidentate at one Mg, and bidentate at two Mg atoms binding motifs were optimized, and energies were calculated to be thermodynamically accessible.9,10 Molecular (associative) adsorption of HCOOH is also reported but is found to be a high-energy structure as compared to dissociative adsorption.9,10 Likewise, CH3OH can adsorb on the MgO surface in both molecular and dissociative pathways.10 In both studies, activity is enhanced by the large surface area and by defects of the catalyst surfaces. Some experimental studies have been reported on surface reactions of lactic acid on the MgO surface. MgO acts as base as well as a surface to enhance polymerization reactions. The surface reaction is found to be more effective at high surface area particles at the nanoscale level.11 Although bulk MgO is relatively inert, nanocrystalline MgO with surface areas on the order of 250−500 m2/g exhibit enhanced reactivity.12 High surface area MgO has recently been used as a catalyst for the polymerization of lactic acid, and the structural properties of the resulting polymer differ from those of conventionally prepared PLA.11 Because of the number of edge, corner, and defect sites available in nanocrystalline MgO, multiple reactions are possible in the initial reaction stages. An understanding of LA−MgO surface reactions could elucidate the species that are involved in the polymerization.

INTRODUCTION Biorenewable and biodegradable polymers are of great interest as a way to provide alternatives to petroleum-derived polymers as well as to address environmental issues.1,2 Polylactic acid (PLA) is one biobased polymer of interest. It has broad applications from the biomedical field to manufacturing industrial products like food packaging.3 The starting materials of PLA are lactic acid and lactide dimer. Lactic acid can be produced from organic grain sources by a fermentation process. Lactic acid polymerizes by the esterification process in an acidic medium as well as on some metal oxide surfaces. Solid base catalysts like alkaline earth metal oxides are used because of their good ability to break bonds when molecules adsorb on the surface. MgO is an ideal system as it has simple cubic structure and has been used as a catalyst in several surface reactions.4−10 Theoretical studies on surface reactions of biomolecules on MgO surfaces are of interest to understand the mechanism of adsorption reactions at the molecular level. Studies of atomic level mechanisms could provide information needed to design an economical method of production for many high value chemicals from hydroxycarboxylic acids. Lactic acid is a prototype molecule of this class of chemicals. To our knowledge, no previous theoretical studies have been performed on the interaction of lactic acid with the MgO surface. In the past, however, several theoretical studies have been performed with ab initio methods to understand the adsorption process of related functional groups and small molecules, including −COOH, −OH, formaldehyde, SO2, NO, and H2O.6−10 Formic acid adsorption has been investigated and © 2013 American Chemical Society

Received: October 3, 2012 Revised: January 2, 2013 Published: January 15, 2013 765

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Figure 1. Optimized PBE/DZP geometries of (MgO)x and their binding energies with respect to the monomer energy.

the monomer MgO energy as B.E. = E[(MgO)x] - xE[MgO]. The binding energies per MgO (B.E./x) of the clusters grow as the size of the cluster increases. The binding energy of the nanorod structure of (MgO)12 is greater than the B.E. of the primitive cubic rectangular form, which suggests that this form may be observed for clusters of this size. Structure of Lactic Acid. Lactic acid has several minimum energy structures. The lowest energy structures differ in the orientation of the hydroxyl group with respect to the trans (1) and cis (2) orientations as shown in Figure 2. The trans

Besides MgO surface reactions with lactic acid, experimental verification of its dissociation on TiO2 surface also illustrated how it interacts with metal oxide surfaces.13 Dissociation of OH and COOH groups occurs at about 250 °C during the decomposition process of lactic acid into CO2 and acetate on TiO2.13 Therefore, understanding the mechanism of adsorption of lactic acid on MgO clusters will help to model the active catalytic surface and the first steps in polymerization. In this study, the interaction of lactic acid with various sizes of MgO is investigated at different levels of theories. Geometry reorientation during the adsorption process at highly coordinated Mg and O sites is observed under certain conditions. Overall, this study provides initial insights into the interactions of oxygen-rich biomolecules on MgO surfaces.

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COMPUTATIONAL DETAILS Geometries are optimized with the Amsterdam Density Functional (ADF)14 and the General Atomic and Molecular Electronic Structure System (GAMESS)15 programs. Density functional theory (DFT) at the generalized gradient approximation level with the Perdew−Burke−Ernzerhof (PBE)16 exchange and correlation functional and either a polarized double-ζ (DZP)17 or a polarized triple-ζ (TZP)18 basis set is employed for all geometry optimizations in ADF. The geometries are also optimized with PBE and with secondorder Møller−Plesset perturbation theory (MP2)19 with polarized triple-ζ valence TZV(d,p) basis sets using GAMESS. Adsorption energies are calculated between lactic acid and (MgO)x. For adsorption of lactic acid on the smallest (MgO)2 cluster, single-point energies are calculated using the high level coupled cluster singles and doubles with perturbative triples (CCSD(T))20 method with the TZV(d,p) basis set at the PBE/ TZP geometries to compare the accuracy of the overall calculations.

Figure 2. Optimized lowest energy structures of lactic acid.

orientation is found to be the minimum energy structure by about 2.1 kcal/mol with PBE/DZP level of theory. The transition state between the two orientations has a 4.2 kcal/mol barrier height (Figure 3), which shows that two geometries can easily interconvert at room temperature. Adsorption of Lactic Acid on MgO Surface. Lactic acid may orient on the MgO cluster in a variety of ways. Dissociative adsorption of a proton bound to oxygen is possible for the carboxylic acid group, the hydroxyl group, and for a simultaneous interaction of the carboxylic acid and hydroxyl groups. Associative adsorption of various functional groups is also possible, although these represent higher energy structures as shown in Table 1. Nine different geometries are found in this work (Figure 4), and calculations of adsorption energies at different level of theories are shown in Table 1. Dissociation of a proton occurs in most of these structures. Relative energies shown in Table 1 suggest that PBE/DZP gives satisfactory results with respect to higher-level calculations, so this level of theory can be employed for larger clusters. The adsorption energy of geometry 3 is off by ∼3.4 kcal/mol as compared to the CCSD(T)/TZV(d,p) energy; however, other CCSD(T)/ TZV(d,p) energies are close to the PBE/DZP energies. In

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RESULTS AND DISCUSSION Structures of (MgO)x. Increasing sizes of (MgO)x (x = 1, 2, 4, 6, 8, 9, and 12) clusters are optimized with PBE/DZP. The clusters have primitive cubic structures except for (MgO)12 in which both rectangular (primitive cubic) as well as nanorod structures are optimized (Figure 1). This nanorod structure was previously found during PBE calculations of formaldehyde adsorption on (MgO)12.9 The binding energies (B.E.) of (MgO)x (x = 2, 4, 6, 8, and 12) are calculated with respect to 766

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addition, MP2/TZV(d,p) energies are also similar to the PBE energies, although they are generally slightly lower than the PBE energies. By analyzing the interactions of lactic acid with (MgO)2, it can be concluded that the most favorable interaction is for the carboxylic acid group to react with the MgO cluster and undergo proton dissociation. The four lowest energy geometries (structures 1−4) exhibit dissociative adsorption of the carboxylic acid functionality on MgO. Geometries 3−7 are structures in which both the hydroxyl and the carboxylic acid functional groups interact with the MgO surface; the proton dissociates from the COOH group in 3 and 4 and from OH in 5 and 6. The dissociative carboxylic acid adsorption is found to be more favorable thermodynamically than the dissociative hydroxy adsorption geometries. In geometry 7, a structure in which the C−H bond dissociates is optimized. The two highest energy structures have associative interactions without any proton dissociation. Geometry 8 represents an associative hydroxy interaction. Associative carboxy adsorption is found to be the highest energy structure 9. These adsorption energies would suggest that most polymerization or other secondary reactions will proceed initially through a dissociative pathway, although a few may access an associative pathway. MP2 and CCSD(T) energies of associative interactions are calculated to be higher than the corresponding PBE energies, which we believe is due to the inclusion of correlation effects in these levels of theory that can better describe dispersive interactions as compared to PBE. As the PBE/DZP level energies are typically less than 5 kcal/mol off from the higher-level energies in most of the cases, this level of theory will be exclusively utilized with larger clusters. Adsorption of Lactic Acid on Various Sizes of MgO. The adsorption of lactic acid is examined on various sizes of

Figure 3. Reaction pathway for conversion of trans- and cis-lactic acid.

Table 1. Relative Reaction Energies (kcal/mol) of Lactic Acid Adsorption on (MgO)2 at Different Levels of Theory geometries

PBE/ DZP

PBE/ TZP

MP2/ TZV(d,p)

CCSD(T)/TZV(d,p)/ PBE/TZP

1 2 3 4 5 6 7 8 9

0 6.5 7.4 10.5 21.3 31.2 46.5 65.8 66.1

0 6.4 7.4 11.7 19.7 32.8 44.4 63.6 62.7

0 5.2 3.8 8.4 16.7 28.0 45.3 67.6 68.0

0 5.2 4.0 10.7 16.7 28.2 46.6 70.8 69.7

Figure 4. Various types of adsorption of lactic acid on (MgO)2. 767

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surface and edge sites than corner sites and lactic acid could potentially bind to these sites as well, it is important to understand how the binding energies are affected by coordination numbers of Mg and O. In this section, lactic acid binding to a number of differently coordinated Mg and O is investigated for the (MgO)12 cluster. This cluster is chosen because it possesses 3-fold, 4-fold, and 5-fold coordinated Mg and O atoms. One such type of interaction, dissociative −COOH binding, is shown in Figure 6. Other types of lactic

(MgO)x clusters. The energies for a particular type of adsorption on low coordinated sites of different sizes of MgO are calculated and listed in Table 2 for selected geometrical Table 2. PBE/DZP Reaction Energies (kcal/mol) of Lactic Acid Adsorption on (MgO)x structure x

1

2

3

5

8

9

2 4 6 8 9 12

−92.9 −69.4 −67.3 −68.3 −56.6 −53.2

−86.4 −49.2 −47.7 −47.5 −73.6 −31.9

−85.6 −67.3 −64.1 −65.0 −21.9 −19.1

−71.7 −71.5 −69.0 −67.2 −56.7 −53.0

−27.2 −24.0 −23.4 −23.7 −23.8 −20.9

−26.9 −23.2 −26.4 −10.6 −24.5 −6.7

structures. The structures are numbered as shown in the previous section. The trends in reaction energies show that, as expected, larger clusters have lower adsorption energies. As for (MgO)2, the most favorable adsorption mode on larger clusters of MgO involves dissociation of the proton on the carboxylic acid group. Figure 5 displays structures for this dissociative

Figure 6. Interaction of lactic acid carboxylic acid group with various coordination of Mg and O. The numbers indicate the coordination number of involved atoms of (MgO)12.

acid interactions are also optimized at differently coordinated sites on the MgO surface and are shown in Figure S6. The adsorption energies and type of adsorption modes are tabulated in Table 3. In this study, adsorption energies decrease with increase in coordination number of adsorption site of Mg in each type of interaction. Other kinds of adsorptions as in Figure 4 are also optimized with differently coordinated Mg and O of the (MgO)12 cluster. The least favorable adsorption mode is associative carbonyl interaction with (MgO)12. Associative OH interactions are the next lowest in energy, followed by combined COOH and OH, OH, and COOH dissociative interactions. The most favorable adsorption mode is the dissociative COOH adsorption; adsorption energies to rectangular (simple cubic) (MgO)12 vary from −67.1 kcal/mol when both Mg and O are 3-fold coordinated (3,3 site) to −44.9 kcal/mol for a 4,4 coordinated site. It should be noted that adsorption at a 4,5 coordinated site has an adsorption energy of −68.4 kcal/mol relative to the rectangular (MgO)12, but this adsorption mode leads to a nanorod structure with a calculated energy of −60.9 kcal/mol relative to the nanorod geometry of (MgO)12. For adsorption at 4,5 and 5,5 coordinated sites, the geometry of the MgO cluster changes into a nanorod-like shape. For the 4,5 site, the geometrical change affects one-half of the structure; for the 5,5 site, the cluster completely reorients to the nanorod. Similar restructuring is also observed for lactic acid adsorption on highly coordinated sites of the (MgO)9 cluster. This nanorod formation could occur because Mg releases its bond with O in the lower layer to coordinate to the adsorbed lactic acid.

Figure 5. Optimized geometries of most favorable dissociative adsorption of −COOH on increasing sizes of (MgO)x.

carboxylic acid adsorption for increasing sizes of (MgO)x clusters. Adsorption for other clusters is shown in the Supporting Information. As the cluster becomes larger, the relative ordering of the different adsorption modes changes. For example, structure 5 is the second lowest energy adsorption mode for (MgO)12 and is only 0.2 kcal/mol higher than structure 1 for this size of cluster. Structure 9 adsorption energies are not greatly affected by MgO cluster size, so this mode becomes more competitive as the cluster size increases. In both structures 2 and 9, (MgO)9 reorients to a nanorod structure so it does not fit with the typical trend of decreasing energy with cluster size. Interaction of Lactic Acid with Differently Coordinated Magnesium. In the previous section, adsorption structures and energies were determined for lactic acid binding to the lowest coordinated sites of (MgO)x, because these are expected to be the most favorable. However, becauase the experimental nanostructured MgO materials contain more 768

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Table 3. Reaction Energies of Lactic Acid Interaction with Differently Coordinated Mg and O Atoms of (MgO)12 coordination no. of Mg

coordination no. of second Mg (if any)

4 3 3 3 4 5 4 3 3 4 3

5

coordination no. of O

type of adsorption carboxylic carboxylic carboxylic

3 4 4 3 5

carboxylic carboxylic carboxylic hydroxy hydroxy+carboxylic carboxylic carboxylic(5) hydroxy(3) hydroxy (associative) carboxy (associative)

4 3 4 5

3 3

resulting geometries

PBE/DZP energies (kcal/mol)

rod rectangular rectangular rectangular to rod rectangular rod rectangular rectangular rectangular rectangular rectangular

−68.4 −67.1 −54.3 −53.0 −52.8 −50.2 −47.1 −46.5 −46.4 −44.9 −31.9

rectangular rectangular

−20.9 −19.1

Nanorod formation is studied in more detail by adding an extra layer to the MgO by making (MgO)18. In this case, the geometry of the cluster did not change to nanorod (Figure 7). This suggests that the geometry of the MgO cluster is not affected in clusters larger than (MgO)12 that have more than two layers.

Figure 7. Optimized geometry of (MgO)18 with −COOH adsorbed lactic acid at 4,5 coordinated Mg and O.

Figure 8. Interaction of lactic acid carboxylic acid group with (MgO)x and their relative energies (x = 12, 15, 16, 18).

As shown in Table 2, the size of the MgO cluster is a key factor for determining adsorption energies of lactic acid. Because the experimental nanostructured MgO consists of hundreds of MgO units, there is a need to ascertain the variation in adsorption energy with larger clusters of MgO than (MgO)12. To shed light on this issue, one more investigation is performed to study the variation in adsorption energy with clusters larger than (MgO)12. An extra layer is added to (MgO) 12 to make (MgO)15 , (MgO) 16 , and (MgO) 18 ; dissociative adsorption modes of the carboxylic acid functionality of lactic acid are optimized for each cluster size (Figure 8). The calculated adsorption energies at the PBE/DZP level of theory are shown. The calculated adsorption energies increase by up to 4.3 kcal/mol in (MgO)15 and (MgO)16 as compared to (MgO)12. However, in (MgO)18 the energy goes down to −55.4 kcal/mol, which can be justified because of a change in coordination number of lactic acid adsorption from 3,3 to 3,4.

This value is in good agreement with the 3,4-coordinated adsorption energy of −53 to −54 kcal/mol for (MgO)12. Thus, it appears that the coordination environment rather than the size of the MgO cluster dominates the effects on the adsorption energy when the size becomes larger than (MgO)12.

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CONCLUSIONS The adsorption of lactic acid on various sizes of MgO clusters has been investigated. Lactic acid may bind after dissociative adsorption of a proton from the carboxylic acid and/or hydroxyl groups or via associative adsorption of carboxy or other functional groups. Dissociative adsorption of the carboxylic acid functionality is more favorable than hydroxy and carboxy group adsorption. MP2 and CCSD(T) relative energies differ by a maximum of 5 kcal/mol as compared to 769

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PBE energies for the (MgO)2−lactic acid cluster. The adsorption energy generally decreases with larger MgO cluster sizes; however, formation of a nanorod structure for (MgO)9 and (MgO)12 increases the relevant adsorption energy. This nanorod formation is not evident in clusters with at least three MgO layers. For clusters larger than (MgO)12, the adsorption energy depends more strongly on the coordination environment of the Mg and O atoms than on the size of the cluster. As expected, the most favorable adsorption occurs at lower coordinated magnesium and oxygen atoms.

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ASSOCIATED CONTENT

* Supporting Information S

Structures of lactic acid adsorbed on (MgO)x (x = 2−12). Structures of lactic acid adsorption at different coordination sites on (MgO)12. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Donors of the American Chemical Society Petroleum Research Fund for support of this research. REFERENCES

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